The performance of gas turbine engines, whether measured in terms of efficiency or specific output, is improved by increasing the turbine gas temperature. It is therefore desirable to operate the turbines at the highest possible temperatures. For any engine cycle compression ratio or bypass ratio, increasing the turbine entry gas temperature produces more specific thrust (e.g. engine thrust per unit of air mass flow). However as turbine entry temperatures increase, the life of an un-cooled turbine falls, necessitating the development of better materials and the introduction of internal air cooling. In modern gas turbine engines, gas temperatures in a high-pressure turbine are hotter than the melting point of the material of the blades and vanes, necessitating internal air cooling of these aerofoil components.
Conventional high-pressure nozzle guide vane (NGV) castings have a mixture of internal impingement cooling, convection cooling and external film cooling. Impingement cooling systems utilise impingement plates adjacent an external wall and have an array of small holes through which coolant flows to strike the inner surface of the external wall. Impingement plates may be either separate plate inserted to the aerofoil or integrally cast.
U.S. Pat. No. 7,600,973B discloses a conventional ‘multi-pass’ cooling system where coolant passes both radially inwardly and outwardly through main cooling passages. This design uses film cooling, internal turbulators and impingement cooling to cool its suction side aerofoil wall. However, coolant films over the suction side wall's external surface are penalizing on performance but they are necessary to make up for relatively inefficient internal cooling.
There is an ever increasing desire to improve cooling of components in gas turbine engines and especially high-pressure turbine vanes and blades. Improved cooling has many benefits including allowing increased gas temperatures and increased overall engine efficiency, increased life of components and/or decreased amount of cooling air that could otherwise be used for increased propulsive efficiency.
It is therefore one of the objects of the present cooling arrangement to improve cooling effectiveness.